In this study the investigators aim to quantitate the extent of cervical and upper thoracic brown adipose tissue (BAT) activation in lean and obese humans via positron emission tomography-computed tomography (PET-CT) in response to the non-specific beta adrenergic receptor (AR) agonist ephedrine. The investigators hypothesise that this pharmacological adrenergic stimulus will result in activation of BAT in these participants, and that this activation will be reduced in obese patients. This study will provide important preliminary information with respect to allowing the investigators to progress with longer trials with specific beta 3 AR agonists.